Spectroscopic investigation of the molecular vibrations of 1,4-dihydronaphthalene in its ground and excited electronic States.

A comprehensive spectroscopic study of 1,4-dihydronaphthalene (14DHN) has been carried out for its ground and S1(π,π*) electronic states using infrared, Raman, ultraviolet, and laser-induced fluorescence (LIF) spectroscopic techniques. The experimental work was complemented by ab initio and DFT calculations. For the ground state excellent agreement between observed and calculated values was attained. For the S1(π,π*) excited state 19 of the vibrational modes were clearly determined and excited vibronic levels for a number of these were also identified. A detailed energy map for the low-frequency modes in both electronic states was established. 14DHN is very floppy in its S0 ground state but less so in its excited state. The floppiness relaxes C2v selection rules for the S0 state.